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The determination of whether a particular geological formation contains produceable hydrocarbon such as oil can be extraordinarily complicated. Initially, it must be determined what, if anything, a sub-surface formation contains. If it contains fluid, it must be determined whether this fluid is water, hydrocarbon, or both. It must then be determined whether it is cost effective to produce whatever hydrocarbons may be retrievable.
One difficulty encountered by the hydrocarbon industry is its need to retrieve a hydrocarbon stream from the ground that contains only a limited supply of water or brine. Thus, although an area may contain adequate hydrocarbons, excessive water may make it unsuitable for production. Resistivity tools have been useful in determining whether water is present in a hydrocarbon-rich formation. However, the mere presence of sub-surface water does not give a full picture of whether there exists producible hydrocarbon. This also depends upon the character of the detected water. Thus, resistivity tools are not ideal because they indicate merely the presence of water, and cannot indicate its mobility. When underground water comes up-hole with the retrievable hydrocarbons it is known as being free, movable, or reducible. Conversely, when the underground water remains down-hole at the time of production it is known as being bound, immovable, or irreducible. Thus, if one cannot determine the mobility of the underground water, many potentially productive hydrocarbon zones with high irreducible water saturation are bypassed because of fear of excessive water production.
One technology that has proved to be helpful in modem formation evaluation is nuclear magnetic resonance (NMR) technology. This technology assists in the control of water production and identification of pay zones with high irreducible (or bound) water saturation. One such NMR tool is the MRIL(copyright), shown in FIG. 1. Also shown is a borehole 150. The MRIL(copyright) apparatus is a centralized device containing a permanent magnet and a radio frequency (RF) pulse generator (not shown). The tool as shown has an outer diameter 110 of 6xe2x80x3 and a length of about 50xe2x80x2. A slim version of the tool (not shown) has an outer diameter of 4xc2xdxe2x80x3. In an 8xe2x86x92 borehole 150, MRIL(copyright) depth of investigation 120 is 4 inches. The tool""s permanent magnet generates a magnetic field of 2500 gauss (5,000 times the strength of the earth""s magnetic field) with a field gradient of 17 gauss/centimeter. When random hydrogen nuclei interact with the applied magnetic fields, measurable signals are produced. The primary field of the permanent magnet aligns the hydrogen nuclei in one direction. The tool then uses its radio frequency generator to pulse a second magnetic field perpendicular to the permanent magnet""s primary field. This RF generator operates at the Larmor frequency to rotate the nuclei 90xc2x0 with respect to the alignment induced by the permanent magnet. After the RF pulse is turned off, the nuclei gradually dephase or disorder, causing the signal to decay exponentially. MILE operates on three close frequencies, which improves the signal to noise ratio and increases the logging speed. The exponential decay time constant for the dephasing of the nuclei is called the T2 time, and the exponential time constant required for the nuclei to return to their initial aligned position is called the T1, time. The T2 time is shorter than the T1 time and has been chosen as the time measured by the current MRIL(copyright) tool.
This T2 time varies from one hydrogen nucleus to another, depending on the location of the hydrogen in the formation. When the hydrogen is located adjacent an underground rock surface, it comprises immovable or bound water. Surface tension holds this water to the rock surface and causes the water to remain downhole. When this bound fluid is affected by the magnetic field of an NMR tool, the rock causes the bound water to have a shorter T2 time. Moveable water, in contrast, lives in the bulk, and not at the surface of a rock. Thus, the T2 time of its hydrogen is unaffected by a rock""s surface and so is longer in duration. In this way, movable water may be differentiated from immovable water based on their respective T2 times.
One problem with the prior art NMR tools, in addition to many other tools such as the TMD (Thermal Neutron Decay) logging tool, is a limited vertical resolution. For example, because logging data may be sampled at xc2xd foot or xc2xd foot intervals, a common vertical resolution of about 10 inches would be desirable for all tools. However, the vertical resolution of the MRIL tool, for example, is only 2 to 4 feet. Therefore, the response of the tool may indicate only a single layer, when in reality, two or more layers exist in the measured region. The challenge is to establish the actual or xe2x80x9ctruexe2x80x9d response at a specific depth when a tool yields a response that may encompass more than one layer of information.
Certain prior art methods exist to improve the vertical resolution of logging tools, but these methods are not directly applicable to tools such as the MRIL tool or any logging tool that is time, as well as depth, based. For example, improved resolution for an MRIL tool is particularly difficult because the tool must detect a changing value such as a hydrogen nuclei decay (or some other time-based measurement) at each depth rather than simply a xe2x80x9csnapshotxe2x80x9d value.
It is, therefore, not possible to resolve with sufficient accuracy multiple thin beds with a thickness less than the vertical resolution of the MRIL or TMD tool. Thus, present technology may not be able to adequately detect and measure thin beds that contain retrievable oil or other retrievable hydrocarbons. A tool or technique is needed to detect and measure these thin underground layers or beds. Ideally, this tool or technique could be used with most or all of the pre-existing oil field technology.
The above problems may advantageously be solved by a method for enhancing measurement resolution. In one embodiment, the method includes (1) obtaining multiple measurement samples at each of multiple positions along a borehole; (2) extracting reference index samples from the set of measurement samples; (3) determining a difference between each of the reference index samples and corresponding modeled index samples; and (4) updating a set of enhanced index samples based on the difference. The index samples are preferably chosen to be representative of the measurement samples obtained at each position, and accordingly, may be selected ones of the measurement samples, or alternatively, may be averages of the measurement samples. The aforementioned modeled index samples may be found from application of a predetermined tool response to the enhanced index samples, which in turn, may be found by iteration. Once the difference has been reduced below some threshold, the relationship between the enhanced index samples and the reference index samples may be used to calculate a deconvolution filter for all of the original measurement samples from the borehole. When applied to the original measurement samples, the deconvolution filter produces measurement samples having an enhanced resolution.
Thus, the present invention comprises a combination of features and advantages which enable it to overcome various problems of prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings.